Fixture and a method for plating contact bumps for integrated circuits

ABSTRACT

This disclosure describes a plating fixture to hold a silicon wafer containing integrated circuits in a metal plating bath. The wafer is coated with photoresist to a thickness equal to the desired bump height and the desired bump locations patterned by standard photolithographic techniques. The wafer is then loaded in the fixture and the fixture placed in the plating bath so that the patterned side of the wafer is facing up and the plating anode is located directly above the wafer. Systems presently on the market have the wafer positioned with the patterned side facing down and the anode located below it, or the wafer faces sideways and the anodes are access from it. These present systems allow air to be entrapped in the pattern of the photoresist, lowering yield by under plating or uneven plating of the bumps on the wafer. This disclosure prevents such yield loss and also allows cleanups on the wafer after it is loaded in the fixture.

This application is a Continuation-In-Part of U.S. Pat. application Ser.No. 037,760, filed Apr. 13, 1987 now Pat. No. 4,861,452.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to integrated circuit device fabrication, andmore particularly, to a fixture for electroplating metal bumps on themetallized circuit patterns on a sxemiconductor wafer.

2. Description of the Related Art

Metal bump contacts may be used in flip chip technology electrically toconnect an integrated circuit to a substrate, and may be used in TAB(tape automated bonding) technology electrically to connect anintegrated circuit to a leadframe. Typically, the bumps areelectroplated onto metallized integrated circuit contacts on the surfaceof a semiconductor wafer at locations determined by a photoresistpattern on the wafer.

A common prior art technique for electroplating bump contacts on waferswas implemented by patterning the face of the wafer with photoresist,covering the backside of the wafer with photoresist or wax to preventplating of the backside, then placing the wafer in a rack which holdsthe wafer vertically in a plating bath while carrying out the bumpplating process.

U.S. Pat. No. 4,137,867 discloses a fixture for bump plating in whichthe backside of a wafer to be plated requires no coating to preventplating of the backside. This fixture enables a cost savings to berealized by reducing the number of processing steps for bump plating.That is, the steps involved in coating the backside of a wafer are nolonger necessary. This is made possible by a fixture which holds a waferface down at the surface of the plating bath. Only the face of the wafermakes contact with the plating bath, while the backside is kept dry bydirecting a flow of nitrogen gas against the backside.

Both of the above prior art techniques result in a lowered yield due toair bubbles getting trapped in the bump vias(the cylindrically shapedcavities in the photoresist where bumps will be plated on the wafer) inthe photoresist on the wafer face. The bubbles displace the platingsolution in the vias and either prevent bumps from being plated wherethe bubbles are, or cause bumps to be plated which have inadequate shapeor height. It is necessary to circulate the plating solution, or bath,during the plating process, and it is extremely difficult to circulatethe bath without generating bubbles. When plating is performed on wafershaving vertical or face down orientations in the plating bath, thepossibility exists that air bubbles will be trapped in the vias. Withthese prior art wafer orientations, thickening the photoresist coatingon the face of a vafer increases the likelihood that bubbles will betrapped in the vias.

When used wsith TAB or flip chip technology, it is desirable that thebumps be tall. Studies, including computer stress modeling, show thatbumps give more stress relief, and thus greater reliability, thanshorter bumps. The height of a wall formed bump is equal to thethickness of the photoresist on the face of the wafer. Since the depthof the vias is equal to the thickness of the photoresist, it is apparentthat deep vias produce tall bumps. The deep vias are more prone totrapping bubbles than are the shallower vias when the vias are on wafersbeing bump plated by one of the prior art processes in which the wafershave either a vertical or face down orientation during plating. Theaspect ratio of a bump (or via) is defined as height (depth) divided bywidth. Studies have shown that when the aspect ratio reachesapproximately 0.4 or greater, many bubbles become trapped in the vias ofwafers plated by the method disclosed in U.S. Pat. No. 4,137,867. Whenthe aspect ratio is less than approximately 0.4, bubbles which rise inthe fixture and touch the face of the wafer can be made to move alongthe face of the wafer to escape at the edge of the wafer by thecirculation of the plating bath. Thus, the bubbles which reach thesurface of the wafer do not become trapped in the vias. When thecritical ratio of approximately 0.4 is reached, the flow of the bath isno longer able to sweep bubbles out of the vias, and plating may beprevented entirely in vias where there are bubbles, or the bumps may bemisshapen.

Another important feature concerning bumps is their planarity. Planaritycan be expressed in terms of the difference in height between thetallest and shortest bumps on a single integrated circuit chip or on anentire wafer. For example, if all bumps on a single chip were exactlythe same height, their top surfaces would lie in a common plane andtheir planarity difference would be zero. If the tallest bump on a chipis 30 microns high and the shortest is 28 microns, then the planaritydifference for the chip is 2 mirons. When the planarity difference islow, planarity is said to be high.

Planarity is important because it influences the yield of goodintegrated circuits at assembly. Methods for making electrical contactwith the bumps, using the TAB of flip chip processes, give maximumyields when planarity is high, and yeild decreases as planaritydecreases. When planarity is low, the likelihood increases that one ofthe bumps will not form a good electrical contact. This is especiallytrue with flip chips since the substrate to be connected electrically tothe bumps has a surface which is substantially planar. Loss of contactwith a single bump on a chip will cause the entire chip to failassembly. Low Planarity can be caused by bubbles being trapped in thebump vias, where the bubbles either cause the plated in vias not havingtrapped bubbles, or prevent bumps from being plated at all. As thenumber of bumps per chip increases, the chance that one of the bump viason a chip will catch a bubble increases.

When wafers are bump plated in a face down orientation, the only knownway to plate tall bumps and at the same time prevent planarity yieldloss is to pattern photoresist which has a low aspect ratio, i.e.,shallow bump vias. To get a tall bump when the aspect ratio is low, thebump must be overplated so that it has a mushroom shape. The head of the"mushroom" is formed by plated metallization which spreads laterallyalong the suface of the photoresist after the plating process has formeda bump as high as the thickness of the photoresist. This overplate, ormushroom head, can lock the photoresist to the wafer, complicating thefinal removal of the photoresist. Also, the bumps can be placed nocloser together than the amount of overplate for two adjacent bumps,limiting the bump density on a chip. For example, if the bump overplateis 1 mil then the bumps must have at least 2 mils separation betweenthen on the wafer.

A further disadvantage of the fixtures disclosed in U.S. Pat. No.4,137,867 is that any presoak or cleanup treatments needed by the wafersprior to plating must be done before the wafers are mounted in thefixtures, since the fixtures cannot easily be moved, if at all. Presoakof cleanup refers to the removal of oxides and the like from the face ofa wafer prior to beginning the plating process itself. If a wafer driesout after cleanup, oxides reform on it surface and the wafer must berecleaned in the cleanup bath. To prevent reoxidation, a wafer must beplaced in the plating bath within approximately ten seconds afterremoval of the wafer from the cleanup bath.

SUMMARY OF THE INVENTION

This invention provides a transportable bump plating fixture for holdinga wafer in a face up orientation in a plating bath. The fixture includesan elastomer pad which contacts the back of the wafer and forms a sealwhich prevents the plating bath from coming into contact with the backof the wafer. The fixture also includes a means for forming a cathodicelectrical connection to the metallization on the face of the wafer, andfurther includes a plating anode disposed above the face of the wafer.The fixture is open to the flow of the plating bath over the face of thewafer and between the face of the wafer and the anode.

The face up orientation of the wafer in the fixture of the inventionprevents bubbles from being trapped in the bump vias, and thuseliminates trapped bubbles as a cause of low planarity or deformedindividual bumps. Tests have shown that the planarity of tall bumpsproduced in the fixture of the invention is substantially higher thanthe planarity of tall bumps produced by the prior art methods.

The fixture of this invention makes it possible for thick photoresist tobe used to form tall bumps straight sides and flat tops with nooverplating, i.e., cylindrically shaped bumps. This allows integratedcircuit devices to be fabricated in which the bumps have good strainrelief. The absence of overplate also allows the bumps to be placed inclose proximity to one another, which means that an integrated circuitchip can have a high density of electrical contacts (bumps) to theexternal world.

The back, of a wafer mounted in the fixture of this invention, isprotected by the elastomer pad from exposure to the plating bath, thusno extra steps are required to protect the back with photoesist, wax orother such applied protective coating.

The fixture of this invention allows a wafer to be mounted in it andthen soaked in a cleanup solution such as water or an acid (e.g.,sulphuric acid) pickling or descale solution for cleaning up the face ofthe wafer by removing oxides. The entire fixture, with the wafer, canthen be removed from the cleanup solution and transported to the platingbath without having to handle the wafer itself. Not having to handle thewafer directly, minimizes the chances of contaminating or otherwisedamaging the wafer, and minimizes the time requires to transfer thewafer from the cleanup bath to the plating solution. Thus, the fixtureprovided by the invention makes it possible to transfer wafers from thecleanup bath to the plating bath with little risk that oxides willreform on the faces of the wafers.

BRIEF DESCRIPTION OF THE DRAWING FIGURE

FIG. 1 is an exploded isometric view of an embodiment of the fixture ofthe invention.

FIG. 2 is a side sectional view of the embodiment of the invention shownin FIG. 1.

FIG. 3 is an exploded isometric view of a useful embodiment of theinvention.

FIG. 4 is a side sectional view of the embodiment of the invention shownin FIG. 3.

FIG. 5 is a side view of another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As seen in FIG. 1, the fixture 1 includes a top plate 2, an anode 3, aframe 4, an elastomer pad 5, three cathode needles 6, and a base plate7. The top plate 2 and base plate 7 are constructed of an electricallyinsulating material such as plastic, and although the frame 4 ispreferably an insulator, it may be electrically conductive if it iscoated with insulation resistant to the plating bath and cleanupsolutions.

The anode 3 is positioned at the top of the fixture 1 and is attached tothe top plate 2 by means of anode screw 8, as can be seen in both FIGS.1 and 2. The positive terminal of the plating power supply is connectedto the anode 3 by means of anode wire 9 and the anode screw 8 atconnection 10. The anode 3 is sized to be the same diameter or slightlyless than that of the wafer 11 to be plated, and the frame 4 sets theanode to wafer distance at the optimum distance for the particularplating process to be used.

The wafer 11 to be plated is placed face 13 up on the elastomer pad 5which is positioned on the upper surface 12 of the base plate 7. Thethree cathode needles 6 are located 120 degrees apart and makeelectrical contact with the wafer 11 just inside the periphery of thewafer. The cathode needles contact the metallization on the face 13 ofthe wafer 11 and are electrically connected to the negative terminal ofthe plating supply by means of cathode wires 14 and cathode connectors15. As indicated in FIGS. 1 and 2, the cathode needles 6 each have oneend anchored in one of the cathode connectors 15 where they makemechanical and electrical contact with the cathode connectors.

The cathode connectors 15 and the cathode needles 6 are electricallyinsulated from the plating bath at 16 and 17 respectively, as seen inFIG. 2. The insulation at 16 and 17 is resistant to the surroundingplating bath. The cathode needle points 18, however, are not insulatedso that they can penetrate the photoresist on the wafer 11 and makeelectrical contact with the underlying metallization on the face 13 ofthe wafer 11. The insulation minimizes the area of the negatively biasedconductors which get exposed to the plating bath so that unwantedplating of the fixture 1 parts is minimized.

In another embodiment of the invention, the frame 4 is electricallyconductive and is covered with electrical insulation which is resistantto the plating bath, in like manner to the cathode connectors 15. Inthis embodiment, the cathode connectors 15 can be eliminated andelectrical connection of the cathode needles 6 to the negative terminalof the plating power supply is achieved through the frame 4. The frame 4can be electrically connected to the plating power supply at a point onthe frame which is at or above the plating bath surface 19.

The cathode needle points 18 exert a spring force upon the wafer 11sufficiently great to ensure good electrical contact of the points withmetallization on the face 13 of the wafer 11. This force also pressesthe wafer 11 against the elastomer pad 5 to form a good seal between theback of the wafer 11 and the elastomer pad 5 to prevent the plating bathfrom coming into contact with the wafer back. The elastomer pad alsofunctions as a cushion to help prevent the force exerted by the cathodeneedles 6 from fracturing the wafer 11. Preferably, the elastomer pad 5is made of a resilient material such as silicone rubber or neoprenewhich is resistant to the cleanup solution and the plating bath.

During the plating process the fixture 1 is immersed in the plating bath20 no deeper than is necessary to ensure that the anode 3 is completelysubmerged in the bath, as illustrated in FIG. 2. As seen in FIG. 2,enough of the top plate 2 is above the plating bath surface 19 to keepthe anode screw 8 and the electrical connection 10 (shown in FIG. ) ofthe anode wire 9 to the anode screw 8 above the plating bath surface 19.In the embodiment of the invention where the frame 4 is electricallyconnected to the plating power supply through the frame 4, the frame isconnected to a wire from the power supply at a terminal which also isabove the plating bath surface 19. Such a terminal can be formed insimilar fashion to anode screw 8, where such a screw would penetrate thetop plate 2 and make electrical contact with the frame 4. In eitherembodiment, the fixture can be held in the plating bath 20 in theposition shown in FIG. 2 by any conventional means, such as a supportunder the base plate 7.

As can be seen in FIG. 1, air holes 21 are provided in the top plate 2to allow air bubbles around the anode 3 to escape during plating. Drainopenings 22 at the bottom of the fixture 1 allow cleanup and platingsulutions to drain off the wafer 11 when the fixture is removed fromthose baths.

In an alternative embodiment, shown in FIG. 3, a drive mechanism 31 iscoupled to fixture 1. Drive mechanism 31 includes a motor 23 which iscoupled through a shaft 24 to fixture 1. Motor 23 can be any suitableelectric or other type motor and is affixed to a baseplate 28 using aplurality of mounting devices 29. Baseplate 28 is affixed to brushholder 27 using a plurality of brush holder mounting devices 30. Aplurality of brushes 26 are mechanically attached to brush holder 27 andelectrically connected to the anode wire 9 and the cathode wire 14.

Fixture 1 as shown in FIG. 3 includes a slip ring holder 32 having aplurality of slip rings 25, a shaft, a top plate 2, an anode 3, and aframe 4. Slip ring holder 32 is attached to top plate 2 which is coupledthrough shaft 24 to motor 23 as described above. Other usefulembodiments could employ a stationary anode 3 because it is notessential that the anode by moving relative to the wafer face 13. Aplurality of slip ring contacts 33 are connected to the the slip rings25 and supply anode 3 through anode wire 9 and cathode needle points 18through cathode wire 14 as described above.

FIG. 4 shows fixture 1 in plating bath 20. Fixture 1 is identical to theearlier described embodiments except that in this case, base plate uppersurface 12 has a depressed area 34 just deep enough to ensure that thewafer face 13 is planar with the base plate upper surface 12 when wafer11 is placed atop an elastomer pad 5 as described in the aboveembodiments.

In operation, a wafer 11 is placed on elastomer pad 5 and the fixture 1placed in plating bath 20. The motor 23 is energized thereby startingrotation of the shaft 24 and fixture 1. Plating current is then appliedthrough the anode wire 9 to anode 3 and through the cathode needlepoints 18 to cathode wire 14 as described for the previous embodiments.

FIG. 5 depicts another useful embodiment in which the plating bath 20 iscirculated using a rotating pipe 36 which rotates a nozzle 37 which isdischarging plating solution. In this embodiment, the wafer 11 is heldstationary and face up in the plating bath 20. It should be noted thatany method of circulating the plating bath 20 so that the wafer isplated in a constantly changing circulation current that changesdirection with respect to the wafer will meet the requirements of thisprocess.

What is claimed:
 1. A method of plating bumps on metallization on theface of a wafer, comprising the steps of:(a) placing a wafer face up ona pad in a bump plating fixture so that the back of the wafer abuts thepad, a face of the wafer to be plated is coplanar a face of a base platein the fixture and the wafer is disposed below an anode in the fixture;(b) placing a cathode needle against the metallization on the face ofthe wafer to effect electrical contact action with the metallization;and (c) creating relative motion between a plating bath and the face ofthe wafer to be plated.
 2. The method claim 1 wherein the step ofplacing a cathode needle against the metallization on the face of thewafer also causes the back of the wafer to press against a pad to form ahigh integrity seal to prevent the plating bath from coming into contactwith the back of the wafer.
 3. The method claim 1 wherein the step ofcreating relative motion between a plating bath and the face of thewafer to be plated includes the step of rotating the fixture.
 4. Themethod claim 1 wherein the step of creating relative motion between aplating bath and the face of the wafer to be plated includes the step ofrotating a flow of plating solution about a normal to the face of thewafer.
 5. The method of claim 1 further comprising the steps of:(a)inserting the bump plating fixture with the wafer into a clean up orpresoak bath; (b) removing the fixture with the wafer from the clean upor presoak bath; and (c) inserting the bump plating fixture with thewafer into the plating bath.
 6. The method of claim 4 wherein the stepof inserting the bump plating fixture with the wafer into the platingbath is carried out with the limitation that the fixture is insertedinto the plating bath only far enough as is necessary to just cover theanode.
 7. The method of claim 1 further comprising the steps of:(a)electrically connecting the anode to the positive terminal of a platingpower supply; and (b) electrically connecting the cathode needle to thenegative terminal of the plating power supply.